1. Field of the Inventions
The inventions are related to brachytherapy in general and are specifically related to the placement and dosage of radioactive materials for brachytherapy.
2. Related Art
Brachytherapy is a type of radiation therapy that involves the placement of radioactive sources (referred to herein as xe2x80x9cseedsxe2x80x9d) either in tumors (interstitial implants) or near tumors (intracavity therapy and/or mold therapy). In this treatment approach, radiation from the radioactive sources is emitted outward and is limited to short distances. Thus, unlike external beam radiotherapy, where radiation must traverse normal tissue in order to reach the tumor, brachytherapy is much more localized and therefore reduces radiation exposure to normal tissue while allowing a higher radiation dose as compared to external beam radiotherapy. Brachytherapy has become increasingly popular for the treatment of early-stage prostate carcinoma; therefore, the present inventions will be discussed with reference to the treatment of a prostate tumor. However, those of skill in the art will recognize that the present inventions are not limited to treatment of prostate tumors and that many other uses of the inventions are possible.
In the past, a major limitation to the use of radioactive seed implants was the difficulty of accurately placing the seeds, which may number from approximately 40-100, in a designated geometric pattern. However, with the advent of imaging devices such as transrectal ultrasound (TRUS), it has become possible to image both the prostate and the radioactive seeds. This in turn allows a radiation oncologist greater control in the placement of seeds than had been possible. Seed implantation is commonly performed with a template 300 (a plastic slab with a rectangular grid of holes in it as shown in FIG. 3), which is attached to a TRUS transducer 410 and mounted on a transperineal implantation device 400 as shown in FIG. 4. The TRUS transducer 410 transmits images to a dedicated display unit. A series of transverse images are taken through the prostate, and the TRUS unit displays the template grid superimposed on the prostate image. Needles inserted at the appropriate grid positions enable seed placement in the target at planned locations.
The existence of a suitable procedure for accurately placing seeds raises a second issue: determining the optimal placement (also referred to as the configuration) of the seeds. The placement of seeds should be chosen to satisfy two criteria: a) the sufficiency of the radioactive dose received by the tumor; and b) the minimization of the radioactive dose received by surrounding healthy tissue. The large number of potential configurations means only a small fraction of configurations can be investigated manually.
A number of prior art techniques used to determine seed configurations have been used in the past, including the Manchester Paterson-Parker system, the Quimby system and the Paris system. One problem with these known methods is that they take a large amount of time (on the order of four hours or more) to perform. Thus, treatment strategies devised using these methods are typically generated in a simulation session several days (or weeks) before placement is to be performed. Unfortunately, it is often the case that the position of the diseased organ in the operating room differs from the position of the organ for which the treatment plan was intended. In such cases, it may be necessary to change the plan in the operating room. What is needed is a method and apparatus for quickly (i.e., within minutes) calculating a good brachytherapy treatment plan.
The aforementioned need is met to a great extent by the present invention which provides an integer linear programming model for the placement of seeds and several techniques for finding optimized solutions for seed placement problems based on the model. The model uses binary (referred to herein as xe2x80x9c0/1xe2x80x9d) indicator variables to represent the placement or non-placement of seeds in a predetermined three-dimensional grid of potential seed locations. In preferred embodiments, the three dimensional grid of potential locations corresponds to the intersections of the rectangular grid of holes of the template discussed above with each of a number of parallel xe2x80x9ccutsxe2x80x9d of the tumor and surrounding tissue imaged by an imaging device such as a TRUS or CT scanner. The images generated by the imaging device are discretized into a number of image points at a granularity which may or may not be equal to the granularity of the template. The dose delivered to each image point is modeled as a linear combination of the indicator variables. A system of linear constraints is imposed to attempt to keep the dose level at each image point within specified bounds. Branch-and-bound and genetic algorithms are provided to find optimized solutions based on the model. The branch-and-bound and genetic methods may either maximize the sum of rewards associated with achieving the specified bounds or minimize the sum of penalties associated with deviating from the desired bounds.
According to one aspect of the invention, techniques for planning the placement of seeds for a brachytherapy treatment of diseased tissue includes representing a placement or non-placement of a seed in each point of a predetermined three dimensional grid of potential seed locations with a binary indicator variable. A tumor and surrounding tissue are represented as a predetermined three dimensional tissue grid having a plurality of tissue points. At least one of an upper bound and a lower bound for a dose of radiation received is associated with each point in the tissue grid. An objective value is calculated based on a difference at each point of the tissue grid between an amount of radiation based on a trial placement of seeds and the upper bound or the lower bound of both. The trial placement of seeds is varied to obtain an optimal value for the objective value. A planned placement of seeds is set based on the trial placement of seeds that obtains the optimal value for the objective value.
According to one embodiment of this aspect, the tumor and surrounding tissue are represented based on biological imaging. According to another embodiment, a larger upper bound is associated with fast-proliferating tumor cells than with slowly-proliferating tumor cells.
According to another embodiment, the tissue grid represents the tumor and surrounding tissue at a particular time. In some of these embodiments, the three dimensional grid of potential seed locations at a time of seed insertion is mapped to a new grid of potential seed locations at the particular time.